1. Field of the Invention
This disclosure relates to solid state relays, and in particular to a control circuit for an optically-coupled solid state relay.
2. Description of the Related Art
Optically-coupled solid-state relays (SSR) are applied to perform switching in telecommunication, battery powered devices, programmable controllers, electronic instruments, and industrial controls, such as microprocessor control of solenoids, lights, motors, etc. Such SSRs may replace electromechanical relays to provide improved switching speeds and reliability of operation.
Many control circuits for optically-coupled SSRs require a photodiode array (PDA) to charge not only the gate capacitance of an output transistor but also to bias a turn-off circuit in an OFF state. Because of the limited current available from the PDA, such charging of the output transistor is relatively slow and therefore reduces switch speed. In addition, large turn-on currents (on the order of mA) are generally required to bias the turn-off circuit in an OFF state.